(1) Field of the Invention
The present invention relates to high voltage porcelain insulators having an excellent steep wave front characteristic.
(2) Related Art Statement
With recent increases in capacity and voltage of power transmission lines, it is expected that the mechanical strength and electrical performances of suspension insulators, improved and and the like, would be that reliability of the suspension insulators, and the like, would be enhanced. In order to raise the voltage in the power transmission lines, it is necessary to enlarge a head portion of the insulators and to prolong a leakage distance of a shed portion thereof, which, however, poses the following problems.
First, enlargement of the head portion of the insulator leads to enlargement and weight increase of metal fittings used and accordingly weight increase in the insulator. Consequently, steel towers become larger and construction cost of power transmission lines increases. Alternatively, it is possible to increase the strength of porcelain by adding a great amount of alumina crystals to the porcelain so that a required mechanical strength may be attained while the shape of the head portion is maintained in a small size. However, since the dielectric strength of the porcelain itself lowers due to increasing of the alumina crystals, electrical performances, such as a steep wave front characteristic, of the insulator accordingly decrease.
When the leakage distance becomes longer, some electrical performances, such as contaminating resistance, are improved. However, since the flashover voltage on the surface of the shed portion becomes higher at lightning, penetration shorting is likely to occur at a porcelain head portion of the insulator. As a result, there is a shortcoming that a steep wave front characteristic is decreased as compared with the case where insulators have heads of the same shape with a shorter leakage distance.
Under the circumstances, there have been desired developments of the insulators which have an excellent steep wave front characteristic even under high voltages while the strength of the insulators themselves are maintained by the same shape as in the conventional insulators.
In general, an insulating portion of an insulator is constituted by a porcelain, a glaze and a sanded portion. Although various improvements on each of these portions have been made, they are all related to improvements in mechanical strength. However, no information has been available concerning the improvements on steep wave front characteristics. Thus, it has been impossible to accomplish the object of the present invention to develop insulators having excellent steep wave front characteristic from the above-mentioned prior art.
Further, in order to improve the dielectric strength, that is, the steep wave front characteristic of the porcelain, it has been contrived to homogenize and make finer the microstructure of the porcelain finer or reduce the dielectric loss of the porcelain. In the former case, the shaping workability is deteriorated and cost largely increases, while in the latter case, there is a limit for the use of the alumina crystals. Further, these countermeasures are essentially contrary to the objects to maintain the production cost or the mechanical strength. Thus, the insulators which met all the requirements for the steep wave front characteristic and mechanical strength could not be obtained only by improving the porcelain itself.
With respect to the glazes used in porcelain insulators, for instance,, Japanese patent publication Nos. 49-21,865 and 51-4,275 disclose insulators in which the coefficient of thermal expansion of the glaze is specified. However, they are all to improve the mechanical strength of the insulators.
Furthermore, with respect to the sanded portions, in EPRI literature "Improvement of Mechanical and Electrical Strength of Porcelain Insulators", it is described that the mechanical strength of the insulators is improved by specifying the coefficient of thermal expansion of the sand. However, this reference is also merely to improve the mechanical strength of the insulators as in the former cases.